tetrarch (n.)

late Old English tetrarche "ruler of one of four divisions of a kingdom or province," from Late Latin tetrarcha, from tetrarches, from Greek tetrarkhes "leader of four companies, ruler of four provinces," from tetra- "four" (from PIE root *kwetwer- "four") + arkhein "to rule" (see archon). Applied generally to subordinate rulers in the Roman Empire, especially in Syria. Related: Tetrarchy.